If a head gasket is failing, coolant under high pressure is getting into the cylinders. The pressure in the cylinders alternates between low pressure during an intake phase of the cycle, and very high pressure during the power stroke. If excessive coolant gets into the cylinder, it can soak the plug, and you get no spark. Okay, you get a misfire (and probably a CEL if it continues). Coolant in the cylinder will mean getting coolant into the oil - the contaminated oil in an engine with a bad head gasket looks like cafe mocha. If even more coolant gets into the cylinder, you get into trouble with spontaneous engine stalling - something that can occur at an unexpected time and in an unexpected place.

A long time ago, when we lived in California, I had a head gasket blow on a Fiat as were climbing Mt Tam, north of SF. Coming home, the car stalled on the San Rafel bridge, where there is no shoulder. Fortunately for us, there was little traffic, and the people bearing down on us at speed were able to change lanes. I have never been so scared driving, and that was about 25 years ago.

Associated with head gasket problems are overheating issues that are expensive to fix - a warped head will require machining for the engine to work properly again, a cracked head will require replacement. The take home message is to get the gasket replaced now, without regard to inconvenience, because putting it off can cause lots of damage to the engine, or it can put you in terrible situation in traffic.